Xylose is a 5-carbon sugar of wide-scale commercial interest, primarily because it may be easily hydrogenated to xylitol, which is used as a specialty sweetener in the food, drug, and confectionary industries. Xylan, a natural occurring polymer of xylose, commonly referred to as a pentosan, and one of the five principal components of hemicellulose, can be converted to xylose by hydrolysis. Therefore, extraction of xylan from cellulosic fibers is of interest.
The dry weight components of arboreal wood, such as hardwood pulps from sources including sweet gum, black gum, maple, oak, eucalyptus, poplar, beech, aspen, and mixtures thereof, are roughly 30% hemicellulose, 42% cellulose, 25% lignin, and 3% wood extractives. The exact quantity of each component varies between species, and within a given species depending on the age of a tree, where it grows, etc.
Hemicelluloses are linear polymers composed of cyclic 5-carbon and 6-carbon sugars (polysaccharides). There are five main classes of hemicellulose, namely galactoglucomannan, arabinoglucuronoxylan, arabinogalactan, glucuronoxylan, and glucomannan. In hardwood species, 75-95% of the hemicellulose is of the glucuronoxylan type. In its native state, hardwood hemicellulose has an average degree of polymerization (DP) of approximately 200, and 80-90% of the principal monomer components are anhydrous D-xylose units.
Cellulose is the main component of wood, contributing 40-45% to the total dry mass. Cellulose is located almost entirely in the cell wall of wood fibers. Like hemicellulose, cellulose is a linear polymer. However, the DP of cellulose is much higher, typically between 1,000 to 10,000, and cellulose chains are composed entirely of anhydrous D-glucose units.
Lignin is a network polymer composed of phenyl-propane monomers, namely ρ-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which are generally referred to as cinnamyl alcohols, and are commonly called lignin C9-units. It contributes to approximately 15% to 35% of the dry mass of softwoods, hardwoods, and woody grasses. Lignin is deposited between individual wood fibers and acts as an intercellular adhesive, binding individual wood fibers together.
“Pulping” is the process of chemically or mechanically liberating the individual cellulosic fibers in wood. In North America, the kraft cooking process is the predominant pulping process although there are other pulping practices, such as sulfite pulping, soda/AQ pulping, solvent pulping, mechanical pulping, and the like, which are all well known in the art. The kraft process is a chemical pulping process where chipped wood is “cooked” or digested in a high temperature broth of sodium hydroxide and sodium sulfide cooking liquor. During cooking, lignin and hemicellulose macromolecules are fragmented and solvated, thereby breaking the intercellular adhesive between wood fibers and allowing separation of a pulp extract stream from the cellulose pulp.
Conventional methods of obtaining xylan involve the pulping of wood to separate the lignin and hemicellulose from the cellulose of the wood fibers, and, thereafter, separating xylan from other pulp extracts. However, the traditional steps required to separate xylan from wood lignin, and other non-xylose containing pulp extracts, requires costly and complex purification steps. Prior art purifies xylan using a precipitation step, and/or the xylose produced by hydrolyzing xylan is purified by chromatographic separation. For instance, Hyatt et al., U.S. Pat. No. 6,057,438, discloses a method of combining the effluent from multiple stages of a hardwood cooking process and recovering xylan therefrom. However, recovery of xylan requires an alcohol precipitation step due to the amount of non-xylan organics in the effluent that must be separated from the xylan. Meleja, et al., U.S. Pat. No. 4,075,406, discloses a method of recovering xylose from pentosan containing raw materials by hydrolyzing the raw materials and then purifying the hydrolyzate. However, the hydrolyzate must afterward be subjected to chromatographic fractionation in order to separate xylose from the solution. Heikkila, et al., U.S. Pat. No. 5,084,104 similarly discloses a method of hydrolyzing a pentosan containing raw material and then purifying the hydrolyzate. Likewise, Heikkila requires that the hydrolyzed material be chromatographically separated to obtain pure xylan. As mentioned, xylose separation stages such as alcohol precipitation and chromatographic separation are costly and undesirable.
It is desired to provide a method of utilizing the xylan content of hemicellulose in a caustic extraction solution to make commercial grade xylose without the necessity of the costly and cumbersome pentose separation steps required in the past, namely alcohol precipitation, chromatographic separation, and the like. It is further desired to integrate the method of utilizing xylan, for the production of xylose, with current pulp processing techniques that simultaneously produce paper-grade or chemical-grade cellulosic fibers.